1. Field of the Invention
The present invention relates to a device for locating an object situated close to a detection area.
It applies particularly, but not exclusively, to the construction of a flat transparent keyboard which can be placed on the display area of a display system such, for example, as a cathode ray tube, a liquid crystal display LCD, etc. . . .
Generally, numerous devices of this kind have been proposed comprising a series of detection zones suitably distributed in the detection area and which are scanned successively by means able to identify and so locate the zone which, at a given moment, detects the presence of an object.
Such detection may take place from the measurement of the variation of an electric parameter associated with each zone. The location of the object is then obtained through identification of the zone concerned by said variation.
A drawback of the above mentioned devices is that they have a number of control points limited to the number of detection zones which it is possible to insert in the same detection area, so as to be compatible with the usual dimensions of an operator's finger and means used for avoiding several detection zones from being influenced at the same moment by the same finger of the operator.
2. Description of the Prior Art
To try to overcome this drawback, a multiplexed type detection device has been proposed, particularly by the patent U.S. Pat. No. 5,016,008 which claims the priority of the french parent patent FR 87 07323 filed on 25th May 1987, in the name of the Societe Francaise d'Equipements pour la Navigation Aerienne (SFENA), in which the locating points consist in the center of gravity of a plurality of detection zones influenced at the same time by the finger of the operator, this center of gravity being defined by using, as weighting coefficient assigned to the center of each of these zones, the level of the variation of the parameter detected for this zone.
It has proved however that the resolution obtained with this solution, although very much better than the previously proposed solutions, is limited because of the number of connections required (at least one separate connection per detection zone).
Furthermore, the conventional solution consists in using multi-layer printed circuits, as described in said patent, which increases the number of detection zones, but which cannot be used for forming flat transparent keyboards. In fact, because of the multiplicity of zones and the electrically conducting connections as well as the relatively great thickness of the printed circuits, it is practically impossible to obtain a sufficiently transparent assembly.